Multi-line varieties, mixtures of rice varieties and near-isogenic lines (NILs) with different complete resistance genes have been reported to be effective in controlling blast disease in Japan (Shindo and Horino 1989; Koizumi 1994; Koizumi and Fuji 1994; Nakajima et al. 1996). To develop a multi-line variety that is adapted well to the tropical zone, we introgressed

nine different kinds of single blast resistance genes - Pik, Pi7(t), Pi3, Pi5(t), Pita-2, Piz, Piz-5, Pish, and Pi9(t) - into an Indica-type genetic background, specifically, line IR49830-7-1-2-2, a rainfed lowland rice with submergence tolerance. Backcrossing was carried out at least six times, and nine kinds of NILs for targeting these seven resistance genes were developed. These NILs are already at the BC6F11-BC6F13 generation in 2004. They were designated as IRBL, followed by the resistance gene and an abbreviation for the resistant donor and /RL (for rainfed lowland) (Table 1).

The resistance genes in the IR49830-7-1-2-2 genetic background were investigated based on a differential system, which consisted of differential varieties (Tsunematsu et al. 2000) and 19 standard blast isolates (Pyricularia grisea Sacc.) from the Philippine (Yanoria et al. 2000). At least four genes -Pia, Pib, Pik-s, and Pita- were detected. Genetic analyses based on allelism tests with the F₂ populations derived from the crosses between IR49830-7-1-2-2 and differential varieties confirmed the presences of these genes (data not shown).

Each NIL was investigated through its reactions against the standard blast isolates, and the expected results were shown. Two NILs, IRBL9-W/RL and IRBLz5-CA/RL, were broadly resistant as they showed resistance reaction to all the isolates. Moderate resistance to several isolates and susceptibility to a single but different isolate were exhibited by IRBLz-Fu/RL and ILBLsh-T/RL. The reaction pattern of IRBLk-Ku/RL was the same with IRBL7-M/RL, and these two NILs could not be differentiated based on the differential system using the blast isolates collected from the Philippines by Tsunematsu et al. (2000) and Yanoria et al. (2000). Two NILs, IRBL3-CP4/RL and IRBL5-M/RL, also showed the same reaction patterns (Table 1). The presence of introgressed resistance genes were also confirmed using by allelism test in each NIL (data not shown).

The NILs are almost fixed for some of the morphological traits examined such as heading date, culm length, panicle length, and panicle number, and showed similarities with those of IR49830-7-1-2-2. Submergence tolerance of the NILs were also investigated and almost NILs were considerably similar with the recurrent parent IR49830-7-1-2-2 except for two NILs of Pi5(t) that showed lower tolerance. (data not shown). Each NIL carried at least five genes and was suitable for enhancing the blast resistance in rainfed lowland rice ecosystem as a multiline variety.

Acknowledgment

This study was carried out under the IRRI-Japan Collaborative Research Project (Phase III and IV) donated from Ministry of Agriculture, Forestry, and Fisheries, and Ministry of Foreign Affairs of Japan.

Reference

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